274 PHOTOSYNTHETIC SYSTEM 



External agents other than light may induce far-reaching changes in 

 the arrangement of the chloroplasts. Mechanical shock or injury, 

 defective water-supply, fluctuations of temperature and chemical stimu- 

 lation may all briug about a transition from the epistrophic to the 

 apostrophic, or even to the systrophic condition. Thus Kraus has 

 shown, in the case of evergreen leaves, that the chloroplasts congregate 

 in a mass at the base of each palisade-cell during winter ; the same 

 statement applies to the green cortical parenchyma of many woody 

 plants. Finally, it must be noted that darkness may also induce 

 apostrophe, especially when its action is prolonged. 



So far as is known, the various movements of chloroplasts are not 

 due to any power of active locomotion on the part of these bodies, but 

 depend upon passive transportation by the general cytoplasm or by 

 specialised cytoplasmic structures. According to Knoll and Linsbauer, 

 for example, the chlorophyll corpuscles in the leaf of Funaria, are 

 drawn to and fro by the strands of a protoplasmic reticulum, which 

 does not, however, represent a permanently differentiated structure. 143 

 Whether chloroplasts may not be possessed of some power of independent 

 amoeboid movement in special cases, particularly among Algae, is uncer- 

 tain. But even if the actual movements of the chloroplasts are always 

 passive, it is at least highly probable that the perception of the stimuli 

 which induce the movements is located in these bodies and that they 

 are, in particular, very sensitive to photic stimulation. As to the 

 manner in which the chloroplasts in their turn incite the cytoplasm or 

 the special cytoplasmic motor organs to carry out the requisite move- 

 ments, we are absolutely without information. 



3. Relations between chlorophyll-content and photosynthetic activity. 



The photosynthetic capacity of a cell must clearly be to a very large 

 extent dependent upon the number of chlorophyll corpuscles that it 

 contains. Palisade-cells take the first place in respect of chlorophyll- 

 content, a circumstance which in itself suffices to characterise these 

 cells as the special photosynthetic elements of the plant. Since the 

 mesophyll of the majority of leaves is distinctly differentiated into 

 palisade-layers and spongy parenchyma, it becomes a matter of special 

 interest to compare these two types of" tissue with reference to their 

 chlorophyll-content, and so to obtain a criterion for a comparison of their 

 respective photosynthetic activities. With this end in view, the author 

 has made rough determinations by actual counting of the average 

 number of chloroplasts present per cell, in these two tissues, in the case 

 of various leaves. The results of some of these estimations may be 

 stated as follows. In the leaf of Ricinus communis there are, on an 

 average, 36 chlorophyll corpuscles in a palisade-cell, and only 20 in a 



